The present invention relates primarily to a method for removing carbon monoxide (CO) from flue gas generated by oxyfuel combustion of a fuel such as a hydrocarbon fuel; a carbonaceous fuel; or biomass. The method may also be used to remove nitric oxide (NO) from the flue gas. The invention has particular application in the processing of flue gas from an oxyfuel combustion process in a pulverized coal fired power station.
The term “SOx” means oxides of sulfur and includes sulfur dioxide (SO2) and sulfur trioxide (SO3). The term “NOx” means oxides of nitrogen and includes primarily NO and nitrogen dioxide (NO2). NOx may comprise one or more other oxides of nitrogen including N2O, N2O4 and N2O3.
It has been asserted that one of the main causes of global warming is the rise in greenhouse gas contamination in the atmosphere due to anthropological effects. The main greenhouse gas which is being emitted, carbon dioxide (CO2), has risen in concentration in the atmosphere from 270 ppm before the industrial revolution to the current figure of about 378 ppm. Further rises in CO2 concentration are inevitable until CO2 emissions are curbed. The main sources of CO2 emission are fossil fuel fired electric power stations and from petroleum fueled vehicles.
The use of fossil fuels is necessary in order to continue to produce the quantities of electric power that nations require to sustain their economies and lifestyles. There is, therefore, a need to devise efficient means by which CO2 may be captured from power stations burning fossil fuel so that it can be stored rather than being vented into the atmosphere. Storage may be deep undersea; in a geological formation such as a saline aquifer; or a depleted oil or natural gas formation. Alternatively, the CO2 could be used for enhanced oil recovery (EOR).
The oxyfuel combustion process seeks to mitigate the harmful effects of CO2 emissions by producing a net combustion product gas consisting of CO2 and water vapor by combusting a carbonaceous or hydrocarbon fuel in pure oxygen. This process would result in an absence of nitrogen (N2) in the flue gas, together with a very high combustion temperature which would not be practical in a furnace or boiler. In order to moderate the combustion temperature, part of the total flue gas stream is typically recycled, usually after cooling, back to the burner.
An oxyfuel process for CO2 capture from a pulverized coal-fired power boiler is described in a paper entitled “Oxy-combustion processes for CO2 capture from advanced supercritical PF and NGCC power plants” (Dillon et al; presented at GHGT-7, Vancouver, September 2004), the disclosure of which is incorporated herein by reference.
Oxyfuel combustion produces raw flue gas containing primarily CO2, together with contaminants such as water vapor; CO; “non-condensable” gases, i.e. gases from chemical processes which are not easily condensed by cooling, such as excess combustion oxygen (O2), and/or O2, N2 and argon (Ar) derived from any air leakage into the system; and acid gases such as SO3, SO2, hydrogen chloride (HCl), NO and NO2 produced as oxidation products from components in the fuel or by combination of N2 and O2 at high temperature. The precise concentrations of the gaseous impurities present in the flue gas depend on factors such as on the fuel composition; the levels of O2 and N2 in the combustor; the combustion temperature; and the design of the burner and furnace.
In general, the final, purified, CO2 product should ideally be produced as a high pressure fluid stream for delivery into a pipeline for transportation to storage or to site of use, e.g. in EOR. The CO2 must be dry to avoid corrosion of, for example, a carbon steel pipeline. The CO2 impurity levels must not jeopardize the integrity of the geological storage site, particularly if the CO2 is to be used for EOR, and the transportation and storage must not infringe international and national treaties and regulations governing the transport and disposal of gas streams.
It is, therefore, necessary to purify the raw flue gas from the boiler or furnace to remove water vapor; CO; SOx; NOx; soluble gaseous impurities such as HCl; and “non-condensable” gases such as O2, N2 and Ar, in order to produce a final CO2 product which will be suitable for storage or use. It is also necessary to reduce and ideally eliminate emission of impurities such as CO, NOx and SOx into the atmosphere.
In general, the prior art in the area of CO2 capture using the oxyfuel process has up to now concentrated on removal of SOx and NOx upstream of the CO2 compression train in a CO2 recovery and purification system, using current state of the art technology. SOx and NOx removal is based on flue gas desulphurization (FGD) schemes such as scrubbing with limestone slurry followed by air oxidation producing gypsum, and NOx reduction using a variety of techniques such as low NOx burners, over firing or using reducing agents such as ammonia or urea at elevated temperature with or without catalysts. Conventional SOx/NOx removal using desulphurization and NOx reduction technologies is disclosed in “Oxyfuel Combustion For Coal-Fired Power Generation With CO2 Capture—Opportunities And Challenges” (Jordal et al; GHGT-7, Vancouver, 2004). Such process could be applied to conventional coal boilers.
US 2007/0122328 A1 (granted as U.S. Pat. No. 7,416,716 B1) discloses the first known method of removing SO2 and NOx from crude carbon dioxide gas produced by oxyfuel combustion of a hydrocarbon or carbonaceous fuel, in which the removal steps take place in the CO2 compression train of a CO2 recovery and purification system. This process is known as a “sour compression” process since acid gases are compressed with carbon dioxide flue gas. The method comprises maintaining the crude carbon dioxide gas at elevated pressure(s) in the presence of O2 and water and, when SO2 is to be removed, NOx, for a sufficient time to convert SO2 to sulfuric acid and/or NOx to nitric acid; and separating said sulfuric acid and/or nitric acid from the crude carbon dioxide gas.
It is also known generally to oxidize CO and NO to CO2 and NO2 respectively using a range of catalysts. For example, GB 998,771 describes the use of “hopcalite”, a mixture of copper oxide and manganese oxide, for oxidizing CO to carbon dioxide (and hydrogen (H2) to water) at 5 to 10 psig (140 to 179 kPa) and 240° C. in a process for the purification of helium. CO2 is then removed using molecular sieves.
US 2003/0153632 A1 describes processes for removing O2 from synthesis gas (“syngas”) by passing the syngas over a catalyst based on metal/metal oxides at a pressure from atmospheric pressure to about 1000 psi (6.9 MPa) or higher and at a temperature of 20 to 600° C. The catalyst facilitates oxidation of CO in the syngas to carbon dioxide using O2 in the syngas.
The use of catalysis for reducing emissions of CO and NO from different forms of air-fired combustion has also been described. For example, GB 411,655 describes the use of a noble metal catalyst (platinum and rhodium) or hopcalite for the oxidation of carbon black and CO to clean the exhaust gas from an internal combustion engine. In addition, GB 2,442,444 A describes the use of hopcalite for also oxidizing NO in exhaust gases. The reaction takes place at atmospheric pressure and at a temperature of 30 to 86° C. It discloses that the humidity of the exhaust gases should be from 0.035 to 0.9 kg/kg dry gas.
Spassova et al (Journal of Catalysis; 185; 43-57; 1999) describe the use of hopcalite catalysts for the simultaneous oxidation of CO and reduction of NO at ambient temperature.
Other catalysts for the reduction of NO to N2 are known and are typically referred to as selective catalytic reduction (“SCR”) catalysts. For example, U.S. Pat. No. 5,260,043 A describes a process for the conversion of NOx and CO to N2 and CO2 respectively in flue gas containing O2, using a metal-exchanged crystalline zeolite catalyst and CH4 as a reducing agent. The reaction takes place at 250 to 700° C. and at 0.5 to 300 atm. (50 kPa to 30 MPa). Ammonia (NH3) has also been used as a reducing gas with SCR catalysts.
US 2008/0038174 A1 teaches a two bed system for SCR of NOx. The first catalyst bed contains a mixed metal oxide and the second bed contains a supported noble metal catalyst. The reference indicates that the catalyst system may be used to remove nitrogen oxide within an exhaust gas generated upon combustion of fuels such as diesel, gasoline, coal and the like. Tests of the catalyst system were carried out on a reaction gas containing 10% O2, 150 ppm each of NO and NO2, 5% water, and N2 for the remainder, and on the exhaust from a 5 L-NA engine.
There is a continuing need to develop new methods and apparatus for removing CO, preferably together with SOx and NOx, from flue gas generated by oxyfuel combustion of fuels, and thereby reduce not only the emission concentration, but preferably also the total amount, of these impurities released into the atmosphere, particularly on an industrial scale.